The impossible to solve overheating problem

For overheat at 50mph;
Either;
There is not enough coolant in the system,
or the coolant isn't what you think it is,
or the coolant isn't circulating properly,
or the rad is not shedding heat properly
or there is another source of heat entering the system, that the system just cannot deal with.
It's just that simple.
Everyone has touched on every one of these points, with good tests to prove each of them.

My money is on magnumdart's test .
The heater core is a bypass,like the bigger elbow. But both send the hot water back into the pump for another go around. I would to his test with both of those bypasses defeated and I would install a gutted stat or a restrictor washer for the test, to speed things up. The pump should empty your rad in a couple of seconds, at idle, making a gusher under the hood. Try not to drown the carb.